Process and device to prepare a mixture for use in manufacturing plastic moulded parts

ABSTRACT

In a process for the manufacture of thick, preferably sheet-shaped, thermoplastic molded parts or molded parts having areas of variable cross sections, at least one part of the plastic material is conveyed as a melt into a heatable and optionally coolable compression mold and is subsequently compressed. Plastic granules are mixed in with the melt before the melt is conveyed into a compression mold. A device to prepare such mixture of melt and plastic granules includes a first section designed as a melt extruder and a subsequent section designed as a mixing extruder. The granules can be fed into the melt between both extruder sections or into the second section.

BACKGROUND OF THE INVENTION

The invention relates to a process involved in the manufacture of thick,preferably sheet-shaped, thermoplastic molded parts or molded partshaving areas of variable cross sections, wherein at least one part of aplastic material is to be conveyed as a melt or molten material into aheatable and optionally coolable mold, e.g. a compression mold or aninjection mold, and therein is compressed or injection molded.

Such a process is of interest especially for the manufacture of achamber and diaphragm filter plate which includes a thick-walled edgeand a significantly thinner inner area. This shape of chamber anddiaphragm filter plates is necessary for use in so-called filterpresses, since the filter chambers of such filter presses are formed bydepressions in the individual filter plates when the plates are clampedtogether, arranged in a row with their edges lying side-by-side, into amold plate assembly. In so doing, the significantly thicker edges of theplates serves as a sealing surface to prevent suspensions, which arepressed into the filter chamber and are to be filtered, from escaping.

The manufacture of such thick molded parts, especially those havingvariable thickness, causes problems because, while cooling in the moldfollowing the manufacture from molten plastic, voids, sink marks in thesurface and stresses that can lead even to crack formation form due tothe large, especially varying material thickness. Therefore, cooling inthe compression mold must be conducted very slowly and carefully underconstant molding pressure, a process that is not only very timeconsuming, but also that requires that the compression mold be occupiedfor such period of time.

To manufacture such molded parts of variable cross section with largelyhomogeneous material joints without the formation of voids, a process aproposed in DE 31 38 858 C3 in which a preform is inserted as atwo-sided flat plate into the open compression mold. Then thecompression mold is closed and the preform is forced to close under heatand pressure, wherein in a first phase little pressure is applied to thepreform in order to melt the preform on both sides by means of theheated compression mold. Then in a second phase that is temporallyadjusted to the melting process of the surface of the preform, thedistance between upper and bottom parts of the compression mold isincreasingly reduced by increasing the pressure. The material meltedfrom both surfaces of the sheet-shaped preform is forced into peripheralareas by means of the pressure of the compression mold and finally fillsout the entire peripheral area. This type of manufacture requires muchtime and technical experience and does not exclude with certainty theformation of voids, sink marks in the surface of the finished moldedpart, and stresses that form and can lead to the formation of cracks.

A process known from DE 31 38 857 has the same goal of manufacturingmolded parts with homogeneous material joints without the formation ofvoids. In this process, a pre-finished partial molding, namely a frameforming a finished edge, and supporting cams in a plate area, areinserted into the compression mold. The cams are intended for the mutualsupport of the filter plates in the mold plate assembly. Subsequently,other cavities remaining in the compression mold are filled with plasticgranules and then compressed with the pre-finished frame forming theedge and the supporting cams while the compression mold is heated. Withthis process the finished molded part is supported to shrink everywhereby the same amount when cooling. Apart from the fact that the process isrelatively time consuming and expensive, stresses and fractures,especially in the area of transition from the plate to the supportingcam and to the peripheral area, cannot be ruled out with certainty.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a device and a processenabling finished molded parts to be manufactured rapidly and reliablywithout the drawbacks described above.

This problem is achieved according to the invention in that before themelt is conveyed into a compression mold, granules of the plasticmaterial are added to the melt. Thus, several advantages are obtainedsimultaneously. Since only a portion of the plastic material has to betransferred in the molten state, whereas the other portion is added asgranules, only a relatively small amount of energy is necessary tomanufacture the mixture of melt and granules. By adding granules to themelt, the temperature of the plastic mixture is already lowered beforethe melt is conveyed into the mold so that the finished molded part canalso be cooled faster in the mold.

An especially flawless homogeneous structure of the molded part isobtained if, when adding the granules, the melt and the granules have atemperature that results in only the surface of the added granulesmelting. Suitable variations of the ratio of quantities of materialand/or the granule size can serve to adjust and control the desiredtemperature conditions. In this manner, on the one hand the granules areembedded flawlessly and without the risk of cracks forming the curingmelt, whereas on the other hand the core areas of the granules thatremain cold provide that the finished molded part cools faster. Thefinished molded part is thus not only cooled from the outside in thecoolable mold but also somewhat from the inside out.

Another special feature of the invention is the fact that, when ready tobe conveyed into the mold, the mixture comprising melt and granules hasa temperature that is slightly above the crystallization temperature orhas a temperature in the partial crystalline temperature range of theplastic material of the melt. With this procedure energy can be savedwhen the finished molded part is rapidly cooled in the, for example,cooled mold, so that a higher output can be achieved.

The mixture comprising melt and granules is manufactured preferably withan extruder or an injection molding machine prior to being fed to themold. In this manner the process of the invention can be carried outwith relatively little equipment cost. It is especially economical ifthe melt is prepared in a first section of the extruder or the injectionmolding machine, and melt and granules are mixed in a second section andthe granules are added between the two sections or in the secondsection.

When using polypropylene as the plastic material, which is especiallysuitable for the manufacture of filter plates, it is proposed with theinvention that the melt be raised to approximately 180° C. to 250° C.and that the granules be added at room temperature to the melt. Thus, itis only necessary to generate energy to melt a portion of the plasticmaterial, whereas a high amount of energy for entirely melting thegranules is not required. However, it is also possible to vary bothtemperatures in order to set an optimal condition. Thus, it is possiblealso to preheat the granules in order to achieve, for example, that at alower melting temperature the surface layer of the granules just barelymelts when the granules are added into the melt. For other plastics thetemperatures can be suitably adapted without further effort.

The process of the invention also offers the advantage that fillers suchas chalk or the like can be added to the melt, when molded parts orareas of molded parts or preforms can be made of a material of lowerquality, such molded parts or areas of molded parts or preforms beingthen jacketed with a plastic layer of higher quality.

The invention also relates to a device to prepare a mixture comprising amelt and plastic granules that are especially suitable for carrying outthe process of the invention described above. The device includes afirst section designed as a melt extruder and a subsequent sectiondesigned, e.g. as a melt extruder with additional mixing properties. Thegranules are fed into the melt between the two sections or into thesecond section. Such a device is simple to build and to operate andresults in the melt and granules being reliably mixed homogeneously, andis suitable in particular for manufacturing molded parts of homogeneousstructure.

Conventional extruders, in which the melt is thoroughly mixed to adegree due to the special shape of screw channels of the extruder screw,are unsuitable within the scope of the invention, where a melt issupposed to be mixed with granules. Therefore, it is proposed with theinvention that the melt extruder with additional mixing properties hasone or more mixing elements that also achieve, e.g. a conveying action,that can also be rotated preferably with a screw of the extruder, andthat have the shape preferably of a disk with openings, lamellae orpins. In this manner the melt and the granules are automatically mixed.Also, spaced over the length of the melt extruder with additional mixingproperties, several of such or identical mixing elements can beprovided. The disk shape of the mixing elements guarantees small spacerequirements. Also, when the mixing elements can be rotated with theextruder screw itself, a separate drive is unnecessary. Preferably, adelivery end of the extruder screw is also designed as such a mixingelement.

An especially optimal mixing of the melt and granules is obtained if,according to a special embodiment of the invention, the mixing elementsof the mixing sections are arranged in depressions of the extruder shaftand have substantially a trapezoidal axial cross section. In this manneran uninterrupted flow of material through the mixing elements isguaranteed. The mixing effect is still further increased if the openingsare positioned, preferably alternatingly, to be sloped relative to theaxis of the extruder. This effect is intensified even more if one partof the openings, as seen in the conveying direction, slopes fromradially inwardly to radially outwardly and another part slopes fromradially outwardly to radially inwardly. In this manner the hot melt isreliably mixed, e.g. in the area of an outer heated cylinder of theextruder and the colder melt is reliably mixed in the core area of thescrew channel, so that a temperature equalization in the mixture isguaranteed.

Furthermore, it is especially advantageous if in the area of the firstmelt extruder the plastic material is preferably progressivelycompressed in the conveying direction and is decompressed in the area ofthe melt extruder with additional mixing properties. Thus, on the onehand the result is that air drawn in with the plastic compound at amaterial feed of the melt extruder is forced out again through thematerial feed, whereas due to the decompression in the mixing extruderan intensive addition of granules is promoted.

To obtain desired decompression following the first melt extruder, it isalso proposed with the invention that the diameter of the extruder shaftin the section of the mixing extruder be smaller than in the end sectionof the melt extruder. Thus, the diameter of the extruder shaft candecrease once again or several times, e.g. step-wise, in the area of themelt extruder with additional mixing properties.

For a reliable manufacture of the desired melt it is advantageous if theextruder screw and/or the extruder cylinder can be heated or cooled, inany case in the area of the first melt extruder.

To take different heat transfers into account, it is especiallyadvantageous in order to obtain a homogeneous melt if the surfacetemperature of the extruder screw can be controlled in such a manner asa function of the temperature of the extruder cylinder that in any casein the section of the first melt extruder the former is approximately15° to 20° above the latter.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features, advantages, and possible applications of thepresent invention will be apparent from the following description ofembodiments thereof, with reference to the accompanying drawings,wherein all described and/or illustrated features form individually orin any logical combination the subject matter of the present invention,and wherein:

FIG. 1 is a schematic vertical sectional view of a device designed as anextruder according to the invention in order to prepare a mixture of amelt and granules;

FIGS. 2a and 2b respectively are side and end views, shown partiallybroken away, of a mixing element of the invention and its arrangement onan extruder shaft; and

FIGS. 2c and 2d are schematic views, shown partially broken away, of twoother mixing elements according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The device shown in the drawings is used to manufacture a mixture thatcomprises a melt and plastic granules and is used in particular in themanufacture of thick, preferably sheet-shaped molded parts or moldedparts with areas whose cross section vary in thickness. In such processat least one part of the plastic material is conveyed as a melt into aheatable and optionally coolable mold (shown schematically at 20) and iscompressed or injection molded. The device is designed, according toFIG. 1, as an extruder, to which the plastic material for themanufacture of melt is fed by means of a feed funnel 11 and to which thegranules to be added are fed by means of a feed funnel 12 with a granulepump. Feed funnel 11 is attached to a stationary input sleeve 13 thatopens into a heatable extruder cylinder 9. Extruder cylinder 9 isreceived in a bearing sleeve 14, which is separated by means of athermal separation 15, e.g. an air gap, from the input sleeve 13. Afirst section of the extruder is designed as melt extruder 1, and asubsequent section of the extruder is designed as melt extruder 2 withmixing properties, i.e. as a mixing extruder. Feed funnel 12 for thegranules is provided in the illustrated case in the upstream or startingregion of melt extruder 2.

For the purpose of manufacturing the melt, within extruder cylinder 9 isan extruder screw 3 that is heated, and in particular in the illustratedcase by means of a hot fluid which is guided, as indicated with arrows,in cylindrical bores 16 arranged in an extruder shaft 6, or in lineshoused in such bores. Extruder cylinder 9 has over its length in essencethe same inner cross section, whereas the diameter of the extruder shaft6 in the heated section of the melt extruder 1 increases in theconveying direction, so that the gap between extruder shaft 6 and theinner wall surface of the extruder cylinder 9 decreases. In this manner,when conveying the melt by driving the extruder shaft 6, the melt isgradually compressed while being conveyed along a screw channel 10 dueto the cross section of the shaft expanding gradually. As a result, airdragged or drawn into the extruder with the plastic compound through thefeed funnel is forced out again via the feed funnel 11. Thus, the meltis conveyed into the second melt extruder 2 in a degasified state.

As is apparent from FIG. 1, the cross section of extruder shaft 6 issomewhat less in the area of chamber Z2 than in chamber Z1 in thedownstream end area of the first melt extruder 1, thus resulting indecompression of the melt. In this area the granules are fed by thegranule pump through the feed funnel 12. In the area of a chamber Z3just before the downstream end of the second melt extruder 2, the crosssection of the extruder shaft 6 once again is decreased slightly forfurther decompression, so that very little heat from friction isproduced.

Shaft 6 has frusto-conical recesses or depressions 7 in the area oftransition from melt extruder 1 to melt extruder 2 and in the area ofapproximately the center of the melt extruder 2. In or on the extrudershaft 6, at depressions 7, are provided disk-shaped mixing elements 5that can be rotated with the extruder screw 3 and that are provided withopenings 4 (the shape of which can be clearly discerned from FIGS. 2aand 2b). Each respective mixing element 5 has a mixing segment 8 havinga cross section of trapezoid shape projecting into the respectivedepression 7 as far as directly into the vicinity of the inner wallsurface of the extruder cylinder 9. Mixing sections 8 have openings 4that are, as is apparent from FIGS. 2a and 2b, distributed over theentire feed surface of the mixing segments 8 and that are, seen in theconveying direction of the extruder, sloped in part from radiallyinwardly to radially outwardly and in part from radially outwardly toradially inwardly. Therefore, in respective mixing zones formed by thedepressions 7 the hot melt is transferred from the cylinder wall towardsthe inside and the colder melt from the core area of the screw channelto the outside. Thus, an intensive thorough mixing and equalization ofthe temperature of the melt with the granules takes place. This mixingeffect is promoted by the respective expansion of chamber Z2 withrespect to chamber Z1 and of chamber Z3 with respect to chamber Z2.Mixing elements 5 also can be fitted with lamellae 17 (FIG. 2c) or pins18 (FIG. 2d) distributed over the circumference of extruder shaft 6.

Another mixing element 5 is provided at the delivery end of the extruderscrew 3. From there the intensively mixed mixture comprising melt andgranules is conveyed directly into a mold. A delivery nozzle can also beprovided that is designed preferably flat, so that the "cold melt", i.e.the mixture that comprises melt and granules and has a final temperatureslightly above the crystallization temperature of the plastic, can beintroduced as a web into the mold. Thus, a specific suitablepre-distribution of the melt-granule-mixture in this form can beachieved. In the subsequent compression operation the "cold melt" isdistributed in the press interior and finally fills it totally in theheated form.

The temperatures of the melt and the granules are adjusted in such amanner with respect to one another that the granules in the mixture aremelted only on the surface of the granules, whereas the core areas ofthe granules are still solid and relatively cold. At the completion ofthe compression operation and the subsequent necessary cooling, forexample in the compression mold, which can be cooled for this purpose, atemperature equalization between the relatively cold core areas of thegranules and the melt enclosing such core areas takes place. Thisresults in the molded parts cooling and curing relatively rapidlywithout the risk of non-homogeneous structures being produced, because acontinuous transition from the molten granule particles to the melt hasbeen created. In this manner a molded part of high homogeneity isobtained, even if the molded part is relatively thick and has crosssections of varying thickness.

I claim:
 1. In a process for the manufacture of thick sheet-shapedmolded parts or molded parts of varying cross sections, said processincluding providing a molten thermoplastic material, conveying saidmolten material into a compression or injection mold and thereincompression or injection molding said material while cooling or heatingsaid material, thereby forming said molded part, the improvementcomprising:prior to conveying said molten material into said mold,adding non-molten granules of said thermoplastic material to said moltenmaterial to thereby form a mixture of molten material and non-moltengranules; and then introducing said mixture of molten material andnon-molten granules into said mold and performing said molding operationto form said molded part.
 2. The improvement claimed in claim 1, whereinsaid molten material and said granules have temperatures such that whensaid granules are added to said molten material only surface portions ofsaid granules will melt.
 3. The improvement claimed in claim 1, whereinsaid mixture has a temperature, when conveyed into said mold, slightlyabove the crystallization temperature of said thermoplastic material. 4.The improvement claimed in claim 1, wherein said mixture has atemperature, when conveying into said mold, in the partial crystallinetemperature range of said thermoplastic material.
 5. The improvementclaimed in claim 1, comprising forming said molten material and addingsaid granules to said molten material in an extruder or an injectionmolding machine, and feeding said mixture therefrom into said mold. 6.The improvement claimed in claim 5, comprising forming said moltenmaterial in a first section of said extruder or said injection moldingmachine, and mixing said molten material and said granules to form saidmixture in a second section of said extruder or said injection moldingmachine.
 7. The improvement claimed in claim 1, wherein saidthermoplastic material is polypropylene, said molten material is raisedto a temperature of approximately 180° C. to 250° C., and said granulesare added to said molten material at a lower temperature.
 8. Theimprovement claimed in claim 7, wherein said lower temperature is roomtemperature.
 9. The improvement claimed in claim 1, further comprisingadding a filler material to said molten material.
 10. In an apparatusfor the manufacture of thick sheet-shaped molded parts or molded partsof varying cross sections, said apparatus including a compression orinjection mold capable of being heated or cooled, and means forproviding a molten thermoplastic material and conveying the moltenmaterial into said mold, whereby said mold is operable to form themolten material into a molded part, the improvement comprising:saidmeans including first and second sections; said first section includingmeans for forming the molten material; means for introducing non-moltenthermoplastic granules into the molten material at a position betweensaid first and second sections or into said second section; said secondsection including means for mixing the molten material and the granulesto form a mixture thereof; and means for conveying the mixture of moltenmaterial and non-molten granules from said second section into saidmold.
 11. The improvement claimed in claim 10, wherein said meansincluding said first and second sections comprises an extruder.
 12. Theimprovement claimed in claim 11, wherein said mixing means comprises atleast one mixing element mounted for rotation in said extruder.
 13. Anapparatus as claimed in claim 12, comprising plural said mixingelements.
 14. The improvement claimed in claim 12, wherein said extruderincludes an internal rotatable shaft having an extruder screw forconveying the material.
 15. The improvement claimed in claim 14, whereinsaid mixing element is mounted on said shaft for rotation therewith. 16.The improvement claimed in claim 15, wherein said mixing elementcomprises a plurality of pins extending radially outwardly from saidshaft.
 17. The improvement claimed in claim 15, wherein said mixingelement comprises a plurality of lamellae extending radially outwardlyfrom said shaft.
 18. The improvement claimed in claim 15, wherein saidmixing element comprises a disk extending radially outwardly from saidshaft and having a plurality of openings.
 19. The improvement claimed inclaim 18, wherein said openings extend through said disk in directionsinclined to an axis of said shaft.
 20. The improvement claimed in claim19, wherein alternate of said opening incline oppositely.
 21. Theimprovement claimed in claim 18, wherein first of said opening inclineradially outwardly relative to a direction of conveyance, and second ofsaid openings incline radially inwardly relative to said direction. 22.The improvement claimed in claim 18, wherein said disk is positioned inan annular recess formed in said shaft and has substantially atrapezoidal axial cross section.
 23. The improvement claimed in claim14, wherein said first section is constructed to progressively compressmaterial during conveyance thereof in a conveying direction, and saidsecond section is constructed to decompress the material.
 24. Theimprovement claimed in claim 23, wherein said shaft in said firstsection has a diameter that progressively increases in said direction.25. The improvement claimed in claim 23, wherein said shaft has adiameter in said second section that is smaller than in an axial endportion of said first section.
 26. The improvement claimed in claim 14,wherein said extruder includes an outer cylinder coaxially surroundingsaid shaft, and further comprising means for heating or cooling at leastone of said cylinder and said screw at least in said first section. 27.The improvement claimed in claim 26, wherein said heating or coolingmeans comprises means for controlling the temperature of said screw tobe approximately 15°-20° C. above the temperature of said cylinder insaid first section.